1. Field of Invention
This invention relates to a process of making metal oxide nanoparticles capable of forming a stable dispersion in a hydrophobic polymer matrix, as well as to such a hydrophobic polymer matrix. The hydrophobic polymer matrix having metal oxide nanoparticles therein can find utility in a broad range of applications, including as a charge transport layer composition of a photoreceptor used in electrophotography.
2. Description of Related Art
In electrophotography or xerography, an electrophotographic plate, or photoreceptor, comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles, for example from a developer composition, on the surface of the electrophotographic plate. The resulting visible toner image can be transferred to a suitable receiving member such as paper.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise at least a substrate support, an electrically conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer. The imaging members can take several forms, including flexible belts, rigid drums, etc. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder.
Examples of photosensitive members having at least two electrically operative layers including a charge generating layer and a diamine containing transport layer are disclosed in U.S. Pat. Nos. 4,265,990, 4,233,384, 4,306,008, 4,299,897 and 4,439,507. The disclosures of these patents are incorporated herein by reference in their entireties.
Photoreceptor wear is an area of concern as wear has a major impact on total cost of the device and a negative impact upon print quality, particularly in color xerographic devices. To combat photoreceptor wear, two main approaches have been taken: the use of reinforced charge transport layers or the use of overcoats.
Charge transport layers are known to be comprised of any of several different types of polymer binders that have a charge transport molecule dispersed therein. However, these conventional charge transport layers suffer from a fast, nearly catastrophic wear rate. Previously, Xerox has invented a reinforced charge transport layer containing polytetrafluoroethylene (PTFE) alone (see, for example, U.S. Pat. No. 6,337,166), silica filler alone (see, for example, U.S. Pat. No. 6,501,934) and PTFE along with silica (see, for example, U.S. Pat. No. 6,326,111), which extend the life of the photoreceptor. However, silica filler in a charge transport layer typically has large porous areas that may cause deletion by trapping ozone and nitroxides.
U.S. Pat. No. 5,096,795 describes an electrophotographic imaging member comprising a charge transport layer comprised of a thermoplastic film forming binder, aromatic amine charge transport molecules and a homogeneous dispersion of at least one of organic and inorganic particles having a particle diameter less than about 4.5 μm, the particles comprising a material selected from the group consisting of microcrystalline silica, ground glass, synthetic glass spheres, diamond, corundum, topaz, polytetrafluoroethylene, and waxy polyethylene, wherein the particles do not decrease the optical transmittancy or photoelectric functioning of the layer. The particles provide coefficient of surface contact friction reduction, increased wear resistance, durability against tensile cracking, and improved adhesion of the layers without adversely affecting the optical and electrical properties of the imaging member.
U.S. Pat. No. 5,725,983 describes an electrophotographic imaging member including a supporting substrate having an electrically conductive layer, a hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, an anticurl back coating, a ground strip layer and an optional overcoating layer, at least one of the charge transport layer, anticurl back coating, ground strip layer and the overcoating layer comprising a blend of inorganic and organic particles homogeneously distributed in a film forming matrix in a weight ratio of between about 3:7 and about 7:3, the inorganic particles and organic particles having a particle diameter less than about 4.5 μm.
Thus, attempts to utilize particles in outer layers of a photoreceptor in an effort to increase the hardness/durability of the outer photoreceptor layers have been made. However, these particles have been difficult to disperse uniformly in the materials typically used for certain layers of the imaging member, particularly the charge transport layer. When a charge transport layer is formed from a dispersion in which such particles are poorly dispersed, the imaging member exhibits lesser electrical performance and poorer print quality.
What is still desired, then, is an improved wear resistant photoreceptor, as well as a charge transport layer of an imaging member that forms an excellent dispersion when particle additives, particularly metal oxide nanoparticles, are included in the composition.